And, On Its Return, Forms An Image Of The Slit At D, Which Is Observed
Through The Eye-piece. E Represents The Electric Fork (the Prongs Being
Vertical) Bearing The Steel Mirror M. K Is The Standard Fork On Its
Resonator. C Is The Cord Attached To The Valve Supplying Air To The
Turbine.



The Lens.


The lens was made by Alvan Clark & Sons. It was 8 inches in diameter;
focal length, 150 feet; not achromatic. It was mounted in a wooden frame,
which was placed on a support moving on a slide, about 16 feet long,
placed about 80 feet from the building. As the diameter of the lens was so
small in comparison with its focal length, its want of achromatism was
inappreciable. For the same reason, the effect of "parallax" (due to want
of coincidence in the plane of the image with that of the silk fiber in
the eye-piece) was too small to be noticed.



The Fixed Mirror.


The fixed mirror was one of those used in taking photographs of the
transit of Venus. It was about 7 inches in diameter, mounted in a brass
frame capable of adjustment in a vertical and a horizontal plane by screw
motion. Being wedge-shaped, it had to be silvered on the front surface. To
facilitate adjustment, a small telescope furnished with cross-hairs was